A fusion phage epitope library, consisting of a vast number of foreign peptide sequences (10 to the 9th to 10 to the 11th) expressed on the tip of filamentous phage particles, is being constructed. The foreign peptide inserts will be accessible to bind to antibodies or receptors; those reactive with an antibody of interest can be purified and the amino acid sequence deduced fro DNA sequencing. The entire library of peptides can be screened simultaneously, permitting far more candidate sequences to be screened than is feasible using chemically-synthesized peptides. Unlimited quantities of peptides can be generated inexpensively by growing the phage in E. coli. Two major modifications to commercially-available filamentous phage vectors are essential for construction of the library, and these must be accomplished without disavling the infective capacity of the phage. The first modification, insertion of an antibiotic resistance gene (tetracycline) has been completed for two different phages. The second, reation of a restriction enzyme cutting site in a very precise location within gene III, is currently inprogress. Upon eompletion of the vector, a degenerate set of oligonucleotides will be cloned into this site to produce the initial hexapeptide library. Libraries containing longer peptide sequences, or sequences arranged around known recognition kernels (e.g Arg-Gly-Asp) can be created also. Such libraries will be extremely useful in epitope mapping for the purpose of identifying synthetic peptide vaccine candidates, and identifying functional groups on binding proteins such as hormone receptors and enzymes. This approach utilizes minute quantities of antibody, such as can be isolated from Western blots treated with polyclonal serum.